The impact of increased grid resolution on the mixed layer depth variability in the South Atlantic Ocean and Southern Ocean

The Southern Ocean plays a major role in global climate system. An understanding of Southern Ocean dynamics allows for a better understanding of the carbon cycle and possible future climate conditions. Earth System Models are used to study Southern Ocean dynamics and are currently producing reliable global annual carbon uptake but have limiting seasonal abilities. These models produce dependable results on a global scale, with more conflicting results on a basin scale. Here we study the impact of mesoscale variability on the Mixed Layer Depth in the Sub-Tropical and Sub-Antarctic Zone of the South Atlantic. The region is hugely impacted by the mesoscale variability as a result of the South African boundary currents. We use two regional simulations both at 1/4o resolution, with one model containing online nested child domain over the South African boundary currents (1/12o resolution). The inter-annual simulations both use the same forcing which allow for a comparison study between the two models. Both the nested and standalone model are able to capture the large scale oceanographic features in the domain. The biggest difference is seen in the Agulhas Current region, where the nested model simulates better mesoscale features, resulting in a fairly accurate position of the Agulhas retroflection and return current. The standalone model contains a high temperature and salinity bias which influences the vertical structure of the water column. Both models are able to simulate the seasonality of the MLD in the Sub-Tropical and Sub-Antarctic Zone in the Atlantic sector. The models overestimate MLD in regions closer to the boundary currents. In the nested model the presence of increased mesoscale features promotes stratification of the water column. The differences seen in the MLD of the two models are linked to the temperature and salinity bias in the standalone model as well as the increased mesoscale variability in the nested model.

Reference:

Williams, T. 2016. The impact of increased grid resolution on the mixed layer depth variability in the South Atlantic Ocean and Southern Ocean. University of Cape Town.